A principal step in the process leading to muscle contraction is the intracellular release of Ca2 ion. My overall objctives are (1) to detect and compare physical and chemical events that indicate Ca2 ion release in contracting muscle cells, (2) to describe the effects of agents that are believed to alter intracellular Ca2 ion release during muscle contraction, and (3) to determine the mechanism of excitation-contraction (EC) coupling and its role in influencing the mechanical properties of skeletal and cardiac muscle. Physical and chemical features indicative of Ca2 ion release will be correlated with changes in membrane potential, charge movements (viz., gating currents), and tension development of isolated intact muscle cells. The specific physical features are striation spacing and myofibrillar orientation recorded by visible light microscopy and cine-photography. The chemical feature is the relative change in intracellular (Ca2 ion) recorded as light emission from cells microinjected with the Ca2 ion sensitive photoprotein aequorin. The presence or absence of a correlation among these variables will be used to evaluate the mechanism of action of agents that may change intracellular Ca2 ion release in EC coupling, and to evaluate models for EC coupling (e.g., is spontaneous relaxation during maintained depolarization due to exhaustion of the store of releasable Ca2 ion, or does Ca2 ion release inactivate with time? Are slow non-ionic charge movements unequivocally associated with Ca2 ion channel gating?).